So now one day using my old FP2 (clean A10, see above) instead of my FP4 (A11) I am really excited. If the FP2 would have run so smooth before, I probably had not switched to the FP4 
My setup and settings I described above. I have the new camera modules installed. Besides stock apps I tested Fritz!App TV, CPU-Z and GPSTest. I didn’t test Whatsapp, Signal and no game apps. Under System → Maintenance → Extended Hardware Functions: all enabled
My experience:
- GUI runs smooth and reliable independent of typing or wiping speed. No hangs or freezes.
- battery drain is hard to estimate for me due to bad health of my battery. Nearly zero when the phone is not used
- the stock photo app couldn’t connect to the camera one time, but it could not be reproduced until now
- GPS fixes fast using three Satellite-Systems USA, Russia and China. Galileo is not supported by the FP2 at all as I know. Google Maps positions are exact, compass calibrates fast
- phone calls are o.k. in voice quality and stability. The speaker volume in handsfree mode is low as always before (in media mode speaker volume is much higher). WLan-Call and VoLTE are not available (missing driver support by Qualcomm?)
- sending and receiving SMS o.k.
- browsing with Chrome o.k. including video with Picture in Picture (which crashed in A9), some internet games.
- Bluetooth Box and Headset (cable) connect easily and sound o.k. (BLE activated but not tested)
- Youtube and Fritz!App TV Videos run smooth and reliable
Here is what I found (still) making problems:
- WLan randomly loses connection but reconnects immediately (happens less than once an hour, I notice that from beginning on (A5) together with my Fritz!Box Router; WPA3 is not supported)
- Nightlight definitively doesn’t work! When on, display freezes often especially at video playback. Nothing else helps but Power + Volume down to restart the phone. Maybe it has to do with my display module which is of very first version. I recognize that since introducing nightlight in Android.
Conclusion: with Android 10 Fairphone made the FP2 usable for a longer period of time. Congrats!
Since there were 5 major Android Versions published for the FP2 over the years, I suppose that some problems of my previous versions were based on the consecutive updates OTA via updater. Although I performed more than one factory reset in the past, I think there was something like accumulating waste which led to unreliable functionality. Now the clean install (factory reset A9, manually flashing A10, factory reset again) is something like a rejuvenation for the FP2.